History of Atomic Theory

Democritus (460-370 BC)

460

His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut.”

John Dalton (1766-1844)

Sep 6, 1766

profmokeur.com/chemistry/history_of_the_atom.htm' >John Dalton (1766-1844) was such a brilliant youth that he became an English shool teacher when barely 12 years old. "Daltons theory" All matter is made of atoms. Atoms are indivisible and indestructible.
All atoms of a given element are identical in mass and properties.
Compounds are formed by a combination of two or more different kinds of atoms.
A chemical reaction is a rearrangement of atoms.
Atoms can be neither created or destroyed.

J.J Thomson (1856–1940)

Dec 18, 1856

J.J Thomsons atomic theory model was like a chocolate chip cookie with the postitive charges being the chocolate chips and the negative part is the rest of the cookie. Thomson changed the view on the atom when he found the electron.

Ernest Rutherford(1871–1937)

Aug 30, 1871

<a href='http://http://profmokeur.com/chemistry/history_of_the_atom.htm' > Based on an experiment he had performed - with totally unexpected results - he realized that the atom must have almost all of its mass concentrated at its center, in a nucleus, with the vast majority of the atom consisting chiefly of empty space. They shot alpha particals at a thin layer of gold to study the deflection of angles. He descovered that most the atoms wait is at its nucleus.

Niels Bohr (1885–1962)

Oct 7, 1885

Bohr refined Rutherfords model by proposing that electrons orbit the nucleus without losing energy and could move only in fixed orbits of specific energies. Electrons with low energy would orbit closer to the nucleus while electrons with high energy orbit further from the nucleus.

James Chadwick 1891-1974

Oct 20, 1891

href='http://profmokeur.com/chemistry/history_of_the_atom.htm' >In 1932, James Chadwick identified the neutron. Chadwick was now able to explain the existence of isotopes through his discovery of the neutron. Isotopes of the same element have the same number of protons and electrons but differ in the number of neutrons found in their nucleus.